refmt: "Encountered impossible type specification" error in valid private extensible variant type definition

[jchavarri]

Input (OCaml):

module type S = sig
  type t = private ..
  type t += Foo
end

Output:

refmt: internal error, uncaught exception:
       Reason_pprint_ast.NotPossible("Encountered impossible type specification")

Expected output:

module type S = {
  type t = pri ..;
  type t +=
    | Foo;
};

Related: ocaml/ocaml#1253

This was added in 4.06.0.

[anmonteiro]

Probably fixed in #2036?

The parser is currently written for the 4.04.2 AST